The present invention relates to semiconductor die packages, especially microprocessor packages, and to effective heat dissipation from the packages for achieving high clock-speed targets of microprocessors.
Microprocessor packages that comprise a flip chip die, an integral heat spreader, and a thermal interface material between the back of the die and the integral heat spreader, are known. The thermal interface material plays the critical function of transferring the heat generated by the die to the integral heat spreader lid, which then spreads this heat to other elements such as heat sinks, etc. Heat removal becomes a challenge as the die power consumption, die size and heat density increases with every new generation of microprocessors. Thermal interface materials are used to effectively dissipate heat and reduce thermal resistance of the microprocessor packages. However, a problem in meeting this challenge is high thermal resistance caused by poor thermal interface material capability at end-of-line and/or failures in thermal interface materials post reliability testing. Finding a thermal solution to this problem is critical for achieving high clock-speed targets of microprocessors.
The most commonly used thermal interface materials consist of epoxy resins highly filled with metal or ceramic. These materials, even though they are highly thermally conductive, have significant integration issues with the other components of the package. The high modulus, in the GPa range, nature of crosslinked epoxies leads to severe delamination at the thermal interface material/die and/or thermal interface material/integral heat spreader interfaces which result in high contact resistance due to disruption of the heat conduction path between the die and the lid. Internal stresses are generated from the shrinkage of the polymer upon curing and cooling, and the coefficient of thermal expansion mismatch between the lid and the die. Thermal interface materials with high modulus cannot absorb such stresses which consequently get transferred to the interfaces and dissipated through catastrophic crack propagation mechanisms. Severe delamination can occur at the thermal interface material/die and/or thermal interface material/integral heat spreader interfaces, which result in high contact resistance.
Another important challenge presented by the new generation of microprocessors is to overcome die thin film cracking caused by die peel and shear stresses generated by high modulus thermal interface material in the flip chip/integral heat spreader package configuration. It has been found that thermal greases and phase change materials when used as thermal interface materials cannot meet the performance requirements for packages comprising an integral heat spreader. Greases are limited due to material pump-out during temperature cycling. Phase change materials do not possess high enough bulk thermal conductivities necessary to dissipate the high heats from the new generation of central processing units (CPUs), and they typically require the use of external clamps for the application of a constant positive force for optimum performance. There is a need for a method and package comprising an integral heat spreader, having improved thermal performance. The present invention addresses this need.